For many years it has been common to explore for oil, gas and other valuable minerals by use of seismic techniques. These involve imparting a wave to the earth by, for example, detonating a "shot" of dynamite on the earth's surface or by simply imparting a mechanical vibration to the earth. The wave travels into the earth and is reflected from interfaces separating varying rock layers in the earth's subsurface formation. Detectors spaced some distance from the point at which the seismic energy is imparted to the earth output analog signals upon receipt of the reflected waves. By measuring the time taken by the signal to travel over plural paths to plural detectors, conclusions can be reached about the shape of the interfaces. From analysis of these interfaces, likely locations for deposits of oil, gas and other valuable minerals can be identified.
A perennial problem in the accurate measurement of the time taken by the waves in transit is the recordation of the signals with a sufficiently good signal-to-noise ratio to enable the received signals to be reliably distinguished from noise occurring in the earth and generated by the exploration process itself. In particular, when marine seismic exploration is performed, acoustic microphones, referred to herinafter as "hydrophones", are trailed behind a seismic exploration vessel. The vessel includes means for imparting an acoustic wave to the ocean, which then travels through the ocean and into the sea bed. The wave is reflected from the interfaces between the rock layers forming the sea bed and returns to the detectors streamed behind the exploration vessel. The motion of the "streamer" cable and hydrophones adds substantial noise to the signal. Signal degradation also occurs during transmission of the signals from the hydrophones up the streamer cable to the exploration vessel for recording. The signals are typically converted to digital format for recording; any distortion in the analog signal path or inaccuracy in digitization can be considered "noise." It would obviously be desirable to improve the signal-to-noise ratio of such marine seismic explorations by any means possible so as to allow better identification of geologically significant events in the seismic record.
Presently available analog-to-digital converters are not as advanced as are the data processing methods being used --i.e., the art can decidedly benefit from improved analog-to-digital conversion means. Presently the state of the art in seismic data processing is capable of meaningful analysis of signals of up to approximately 120 dB level difference. The present invention is designed to approach more closely to such sensitivity in encoding analog signals. In particular, the invention is designed to provide more accurate digitization of analog input signals, thus reducing "quantization noise" while eliminating distortion-producing analog circuit components from the signal path. One popular device now in use in analog-to-digital conversion in seismic applications is a gain-ranging amplifier; this shows good dynamic range (sensitivity to a wide range of input signal amplitudes) but poor resolution (insensitivity to small signals superimposed on larger ones). The poor resolution is partly due to serious non-linear distortion in the gain-ranging amplifier.